Devices for measuring microscopic structures are used predominantly in the microelectronic industry for inspecting wafers or masks. Such devices are frequently referred to as "line-width measuring instruments".
The known devices for this purpose are constructed in the manner of a microscope. In other words, the actual measuring instrument is developed either as an accessory to a microscope or as an adaptation of an existing microscope that has been modified for measuring purposes. The known instruments, however, differ in their principle of measurement and can be classified on that basis into three categories.
For a first category, some instruments can post enlarge and focus an optical intermediate image of the microscope onto a tube of a television camera. The actual measurement (of the width of a portion of the structure being imaged, for example) is then accomplished electronically by evaluating the video signal of the television camera. These instruments suffer the disadvantage of being limited to the resolving power of the video camera, so that they are not suitable for highly precise measurements in the submicron range.
The second category includes instruments having a slit diaphragm viewed by a photomultiplier and moved along an intermediate image of the microscope. Examples of instruments operating this way are described in U.S. Pat. No. 4,373,817, issued Feb. 15, 1983, by Coates, and U.S. Pat. No. 4,600,832, issued July 15, 1986, by Grund. Especially high resolution cannot be attained with these instruments either, because it is not practically possible to guide the slit diaphragm precisely and to determine its position accurately, as an element of the measuring process. Furthermore, the scanning speed of this type of instrument is relatively slow.
The second category can also include instruments that displace the object itself in a plane perpendicular to the optical axis, rather than moving a slit diaphragm in the intermediate image of the microscope. The object can be moved for measurement purposes, by means of piezotranslators. This is even more expensive, however, since movement of the object must be effected and measured with a precision that is higher by the linear magnification of the microscope objective--about 40.times. or 100.times.. This puts submicron accuracy out of reach.
The third category includes instruments that do not measure in the object image plane, but move a beam of light focused in a punctiform manner over the structure to be measured. Such instruments are described, for example, in European Patent Application No. EP 0 168 643 A2, published Jan. 22, 1986, by Bille et al., and Federal Republic of Germany Offenlegungsschrift No. DE 36 10530 A1, published Oct. 2, 1986, by Horikawa.
The instruments of these patents employ swivel mirrors or "galvanometer scanners" for deflecting or moving the measuring spot over a structure to be measured. This can produce high measurement speeds, and the measuring spot can be moved rapidly over a relatively large region of a specimen. But for accurate, submicron measurement purposes, speed and range of movement of the measurement spot constitutes a disadvantage, because relatively large movements of the measurement spot are brought about by relatively small changes in angles of deflection of the swivel mirrors. Since the actual measurement is derived from the deflection angles of the mirrors, the tiny deflection angles involved in moving the spot over structures of submicron width limits the resolution of these instruments. Furthermore, galvanoscanners are subject to hysteresis effects that limit the accuracy of their angular measurements. Together, these effects make such instruments unacceptable for submicron measurement.
As explained more fully below, our measuring microscope uses a pivoting plane plate that moves a measuring spot. A plane plate that pivots has been suggested by U.S. Pat. No. 3,941,980, issued Mar. 2, 1976, by Okamoto et al., for adjusting wafers relative to a mask used for their exposure--a purpose quite different from ours. The device of the U.S. Pat. No. 3,941,980 patent is called a "scanning photoelectric microscope"; and its rectangular, plane-parallel prism is pivoted with a uniform speed to move the adjustment marks of the wafer or the mask, as these marks are focused on slit diaphragms in the intermediate image. The signals of detectors arranged behind the measurement diaphragms serve to adjust the wafer precisely with respect to the mask.
For measurement purposes, the device of U.S. Pat. No. 3,941,980 operates like the instruments mentioned in the second category--on the basis of moving the entire object image relative to a measurement diaphragm. Moreover, the pivoting plate serves only for adjusting the wafer position and is not involved in measuring any structure on the wafer itself. Furthermore, like the devices mentioned in categories 1 and 2, this device has the disadvantage that the entire object is illuminated, which reduces the sensitivity of the light detection system and thereby reduces the precision of any obtainable measurement.
U.S. Pat. No. 4,685,775, issued Aug. 11, 1987, by Goodman et al., also suggested a pivoting plane plate cooperating with beam-deflecting pivot mirrors in a laser-scanning microscope, for displacing the laser beam perpendicular to the optical axis. The plane plate in this instrument, however, serves merely to compensate for the travel of the beam off the axis of the pupil of the microscope objective, as caused by the first pivot mirror, since the beam cannot be held precisely at the locus of the pupil. The actual deflecting or moving of the focused laser beam over the structure being measured is accomplished solely by the pivot mirror. Also, no device is included in this microscope for measuring line width, so that the problem we have solved is not addressed by this instrument.
The aim of our invention is to arrange a microscope for measuring microscopic structures with high precision and reliability in the submicron range. We also aim at keeping the measurement device as inexpensive and compact as possible, so that it can serve as an affordable accessory to a microscope.